1. Field of the Invention
This invention relates to a mobile communication system, and more particularly to a mobile station connection method of controlling connection of a telecommunication channel established between a mobile station and a radio base station.
2. Description of the Related Art
In a conventional mobile communication system, when a call is established between a mobile station and a fixed station, speech information transmitted from the mobile station to a radio base station is coded in accordance with the speech coding rule of the radio system of the area in which the mobile station is present. Therefore, the speech information from the mobile station is transmitted to the fixed station after it is converted in accordance with a speech coding rule (for example, 64 kbps PCM (Pulse Code Modulation)) for the fixed station.
Next, connection of a call between two mobile stations will be described. There are various radio systems for establishing channel connecting mobile stations and base stations and as the coding rules suitable for a radio system to code speech information can be selected, there are also various kinds of coding rules.
If the coding rules used by two mobile stations when connecting a call are the same, coded speech information can be transmitted directly to destination mobile station, but if different coding rules are used by two mobile stations when connecting a call, coded speech information cannot be transmitted directly to the destination mobile station.
A method of connecting a call between mobile stations using different coding rules is illustrated in FIG. 1.
FIG. 1 is a view illustrating connection of a call between mobile station 71 and mobile station 72. Mobile station 71 is connected to radio base station 81 by a radio link according to radio system 1 and uses coding rule 1 as a coding rule. Mobile station 72 is connected to radio base station 82 by a radio link according to radio system 2 and uses coding rule 2 as a coding rule.
Mobile station 71 comprises a coder 11 for coding rule 1 and a decoder 12 for coding rule 1. Coder 11 for coding rule 1 codes transmission data in accordance with coding rule 1, and decoder 12 for coding rule 1 decodes received data coded in accordance with coding rule 1.
Mobile station 72 comprises a coder 21 for coding rule 2 and a decoder 22 for coding rule 2. Coder 21 for coding rule 2 codes transmission data in accordance with coding rule 2, and decoder 22 for coding rule 2 decodes received data coded in accordance with coding rule 2.
Radio systems such as TDMA and CDMA systems are available for radio systems 1, 2. As a TDMA system, radio systems such as IS-54, RCR-27 systems are available, and as a CDMA system, radio systems such as IS-95, W-CDMA systems are available.
As coding rules 1, 2, a coding rules such as CELP (Code Excited Linear Prediction) rule is used.
Coding rules are usually different in different radio systems, and in ordinary operation conditions, a mobile station uses a coding rule suitable for the radio system used in the area in which it is present.
For example, when the radio system used is the IS-95 system, a QCELP (Qualcom CELP) rule is used as a coding rule, but when the radio system is the PDC (Personal Digital Cellular) system used in Japan, the VSELP (Vector Sum Excited LPC) rule, whose transmission rate is 8 kbps (full rate), or the PSI (Pitch Synchronous Innovation)-CELP rule, whose transmission rate is 4 kbps (half rate), is used.
Thus, with a conventional mobile communication system, in order to connect a call between two mobile stations which use different coding rules, it is necessary for a radio base station to decode coded speech information from a mobile station of a transmission source into a PCM signal of 64 kbps and then to transmit the speech information to the radio base station of the area in which the transmission source mobile station is present through trunk equipment 90. Further, a radio base station of the area in which a transmission source mobile station is present must code, in order to transmit the speech information to the transmission destination mobile station, the speech information in accordance with a speech coding rule suitable for a radio system of the area in which the transmission destination mobile station is present.
However, in a case like the one described, when speech information is transferred from the transmission source mobile station to the destination mobile station, coding-decoding is repeated twice as coding-decoding-coding-decoding. This caused the problems that coding/decoding delay doubles and the band occupied in the trunk line is longer.
The conventional mobile communication system described above has suffered from the problem that coded speech information cannot be transmitted directly unless the two mobile stations employ the same speech coding rule.
It is an object of the present invention to provide a mobile communication system wherein coded speech information can be transmitted directly from a transmission source mobile station to a transmission destination mobile station of a even when the two mobile stations use different speech coding rules.
In order to attain the aforementioned object, in a mobile communication system according to the present invention, when a request for connection of a call is received, an exchange determines a coding rule or rules to be used by the two mobile stations to be connected. Mobile stations comprises a plurality of coders for coding speech information using different coding rules, a plurality of decoders for decoding coded information using different coding rules, and means for selecting a coder and a decoder to be used in accordance with instructions from an exchange.
According to the present invention, even when speech coding rules used by a transmission source mobile station and a transmission destination mobile station are different, coded speech information can be transmitted directly to the transmission destination mobile station, and consequently, coding/decoding delays can be reduced.
According to an embodiment of the present invention, the coder and the decoder to be used by the mobile station in accordance with the instruction from the exchange utilize the same coding rule.
According to another embodiment of the present invention, the coder and the decoder to be used by the mobile station in accordance with the instruction from the exchange can be selectively a coder and a decoder based on different coding rules.
The present invention allows a suitable speech coding rule to be selected taking into consideration the radio transmission capacity of the area, reducing the band occupied in a trunk line, because a speech coding rule can be designated independently of the radio system of the area in which a mobile station is present.